Widespread application of a semiconductor ultraviolet source with a wavelength of 350 nm or less can be applied in white lighting, sterilization and water purification, high density optical recording light source, various information sensing systems for fluorescence analysis and the like, medical sector, and so on. Thus, development for realizing the short wavelength and high efficiency of semiconductor light emitting devices is being conducted. In recent years, with a light emitting device using a GaN semiconductor material, luminous efficiency at tens of percent has been obtained with an InGaN system in a luminous region where the wavelength is 400 nm or more. Although an AlGaN system is used in a luminous region with a wavelength that is shorter than 400 nm, the luminous efficiency deteriorates drastically in a luminous region with a wavelength that is shorter than 350 nm, and currently external quantum efficiency can be obtained at only several percent. The primary reasons for this are described below.
1) P-type doping of AlGaN system is difficult. Thus, the formation of a pn junction or a pin junction that is essential in forming the light emitting device becomes difficult.
2) Since there is a difference in the crystal lattice constant in the GaN system and the AlN system, with the AlGaN system that is a mixed crystal system of the foregoing systems, deterioration in the crystallinity such as structural defects and penetrative dislocations in the luminescent layer becomes significant.
Meanwhile, a diamond has a broad band gap of 5.47 eV at room temperature, and even under a high temperature higher than the room temperature, it is able to emit deep ultraviolet light with a wavelength of 235 nm by free excitons. Moreover, high carrier mobility has been attained in recent years not only of p-type doping that was considered difficult in the foregoing AlGaN, but also of n-type doping that was considered difficult with diamonds. And even a pn junction with favorable electrical properties in which the rectification ratio is 6 digits or greater has been previously manufactured (S. Koizumi, et. al.: Science 292, 1899 (2001), T. Makino, et. al., Jpn. J. Appl. Phys. 44, L1190 (2005)).
Moreover, since a diamond is configured from a single element, it does not encounter any problems such as structural defects which are unique to the foregoing AlGaN compound semiconductor. In addition, a diamond possesses superior semiconductor characteristics and optical characteristics in addition to favorable mechanical, chemical and thermal properties (with highest thermal conductivity among semiconductor materials). As described above, a deep ultraviolet light emitting device that uses diamond excitons has many advantages over the AlGaN system.
Meanwhile, most of the highly-efficient light emitting devices which have been put into practical use to date are configured from a direct transition semiconductor. The principle of emission with a direct transition semiconductor is the direct recombination of free electron hole pairs with the same symmetrical point (┌ point) of the crystals, which makes the recombination time shorter at an ns order or less. Thus, it is highly likely that the free electron hole pairs will be subject to direct recombination before being captured by the radiative or nonradiative center caused by defects in the crystals. It will possibly increase the internal quantum efficiency to nearly 100% if the radiative or nonradiative center concentration caused by defects in the crystals can be inhibited to a certain degree.
Meanwhile, with an indirect transition semiconductor, free electrons and free holes exist at different symmetrical points in the crystals, and the interposition of phonons is required for recombination. It makes the recombination time roughly 3 to 6 digits longer in comparison to a direct transition semiconductor. Consequently, with an indirect transition semiconductor, it is highly likely that the free electron hole pairs will be captured by the radiative or nonradiative center caused by defects in the crystals before they are subject to direct recombination, and the internal quantum efficiency is only able to achieve a value that is much lower than 1. For the foregoing reasons, some conventional light emitting devices configured from an indirect transition semiconductor used an external emission center employing impurity atoms, and a direct transition semiconductor was primarily used in high efficiency light emitting devices. Since a diamond is also an indirect transition semiconductor, raising the internal quantum efficiency to a practical level is thought to be difficult while maintaining the advantages of 1) and 2) described above as an emission material of a wavelength in the ultraviolet range.